The invention relates to the screen grid calibration of colour picture tubes and in this case is based, in particular, on television sets having colour picture tubes in which the cut-off points are automatically readjusted by means of so-called automatic cut-off circuits. In the case of such television sets, it is customary to use colour picture tubes in which the screen grids (abbreviated to G2) of the three tube systems R, G and B are combined or have a common connection at the colour picture tube base.
Such screen grid calibration, referred to as G2 calibration below, is carried out in mass production of television sets, in particular, after a so-called xe2x80x9cburn-in testxe2x80x9d after the sets have been assembled. In this test the sets are operated as far as possible under operating conditions and this test serves for identifying and filtering out earlier failures. The G2 calibration comprises progressive voltage settings at the screen grids and measurement operations at the control electrodes of the colour picture tube, such as e.g. at the cathodes thereof in the case of the signal driving via the cathodes that is customary nowadays.
The invention relates to simplifying the calibration of grid 2 (G2)in apparatuses having an automatic cut-off circuit for cut-off point regulation of the color picture tube.
In a colour television with a cathode ray tube driven, as is customary, via the cathodes, an inventive arrangement is employed for calibrating grid 2 (G2) operating potential. Calibration of grid 2 (G2) potential is carried out in order to ensure that the video blanking signal VB is present at the cathodes in a specific voltage range in order that the available operating voltage Vcc of the video output amplifier stages can be utilized optimally for the modulation and correct presetting is provided for the cut-off point, called cut-off below, of the respective tube system. In this case, it is not sufficient, however, to set the screen grid voltage to a specific value, since as shown by FIG. 4 with a given screen grid voltage a relatively large cathode voltage range arises on account of variations for the cut-off of the respective tube system and, given a screen grid voltage of e.g. 300 volts, can exceed, or impermissibly restrict, the voltage range available at the cathodes for signal modulation. The voltage range at the cathodes, which forms a so-called modulation window in this case, is bounded by the operating voltage Vcc of the video output amplifier stages, on the one hand, and by the saturation voltage or behaviour thereof, on the other hand.
Therefore, for that cathode which is at the highest potential in the modulation window, a cathode voltage is defined for which the cut-off is achieved by appropriate setting of the screen grid voltage, in order to ensure that, at the respective cathode, the video signalxe2x80x94or the corresponding RGB signalxe2x80x94lies optimally in the modulation range bounded by the operating voltage Vcc and the saturation voltage of the video output amplifier stages. In practice, however, in television sets with cathode control of the colour picture tube and automatic cut-off regulation, instead of the actual cut-off, a cathode voltage level which is known as a quasi cut-off and is defined by the circuit design is set with the aid of the screen grid voltage and corresponds to the test current of the cut-off regulating circuit, in which casexe2x80x94as already mentionedxe2x80x94the setting is oriented to that cathode which, compared with the remaining cathodes, has the respectively highest level with respect to the reference potential. In a cathode voltage level of this type, it is possible to take account of tolerances and/or ageing effects of the picture tube, in order that, during cut-off regulation, there is a reserve, in particular for the occurrence of maximum tolerances and/or ageing effects, which suffices to ensure that the RGB signals cannot be shifted into saturation of the video output amplifier stages, orxe2x80x94in the event of a tolerance- and/or ageing-dictated shift in the other directionxe2x80x94the cut-off points do not drift beyond the available operating voltage Vcc, the consequence of which would be that the tube could no longer be inhibited in the event of black and blanking levels and flyback stripes would thus occur. Consequently, it appears to be necessary to measure the cathode voltages.
For the G2 calibration according to the invention, however, it is possible to avoid measuring the cathode voltages as follows: the relatively large tube-dictated tolerances between the desired cut-off voltage of the cathode respectively at the highest voltage potential and the screen grid voltage to be set for this are in this case not included in the G2 calibration by way of measurement of the cathode voltages, rather a different criterion is advantageously used, namely making flyback stripes visible or the threshold in this respect from which flyback stripes actually become visible. This exploits the fact that, as is known, voltages lying above the respective cut-off are applied to the cathodes during the line and field flyback, in order to ensure that the cathode currents are blanked, that is to say prevented, during the respective time required for this. However, since the signal value xe2x80x9cblackxe2x80x9d usually already lies in the cut-off, in connection with blanking the term xe2x80x9cblacker than blackxe2x80x9d is used. If the screen grid voltage is then increased, the respective voltages between the cathodes and Wehnelt cylinders must also be increased correspondingly in order that the colour picture tube can still be inhibited. If the cathodes reach the maximum available voltage, limited by the operating voltage Vcc of the video output amplifier stages, the colour picture tube can no longer be inhibited when the screen grid voltage is increased further, with the result that flyback stripes become visible.
Therefore, if the screen grid voltage is increased until flyback stripes occur and the screen grid voltage is subsequently reduced until the flyback stripes actually disappear again, it is possible to ensure that the cathode voltage of the cathode ray system whose cut-off is at the respectively highest voltage potential compared with the remaining cathode ray systems corresponds exactly to the maximum available cathode voltage. As a result, the maximum voltage range for signal modulation is available for this cathode.
In order to reliably avoid flyback stripes, however, it is customary to provide a certain voltage amount as a reserve. Furthermore, the ageing behaviour of the colour picture tube has to be taken into account. Due to cathode wear, the electric field strength between the cathodes and the Wehnelt cylinders decreases during the life of a colour picture tube. This wear is compensated for by increasing the cathode voltages with the aid of the automatic cut-off regulation, just called cut-off regulation below. The relationship between the ageing of a colour picture tube and the ageing-dictated shift in the cathode voltage, representing the cut-off, as a result of the cut-off regulation is illustrated in FIG. 3. This means that the screen grid voltage would then have to be reduced again to ensure that the resulting cut-off voltage of that cathode whichxe2x80x94in comparison with the other two cathode ray systemsxe2x80x94is at the respectively highest voltage potential is less than the maximum available cathode voltage determined by a specific amount (that is to say the abovementioned reserve plus a regulating range to be taken into account) (practical values: xe2x88x9210% . . . xe2x88x9215%).
By virtue of the invention, however, settings of the screen grid voltage to the value which corresponds to the maximum available cathode voltage are advantageously avoided. Instead, provision is made for changing over the Wehnelt cylinders of the colour picture tube which are at reference potential to a different or positive voltage potential which preferably takes account of the cathode voltage reserve explained above and of the effect of the cut-off regulation on the signal modulation range. This potential changeover occurs in each case at the beginning of the G2 calibration. Only then is the screen grid voltage set, in which case the operation of making flyback stripes or the like visible on the screen or the visibility threshold in this respect can advantageously be utilized as a setting criterion in this case as well. Afterwards, the potential changeover is reversed and the G2 calibration is thus ended.
The voltage difference to be provided for the potential changeover according to the invention can be determined e.g. empirically or with the assistance of diagrams like the diagrams shown in FIG. 3 and FIG. 4.
The G2 calibration according to the invention thus has the advantage that a single screen grid voltage setting is performed and advantageously measurement at the cathodes of the colour picture tube is rendered unnecessary. In addition there is a significant advantage, in particular for mass production of television sets, in that G2 calibration can be carried out with the rear panel of the TV set housing or case having been mounted.